Cassidy,

I want to add an answer to a question you did not ask. How do you determine the radius and grade of the helix? As Rick said, the diameter is twice the radius. You probably also remember the distance around the circle, the circumference, is 3.14 x diameter.

Now consider the vertical geometry problem for a minute. Depending on how you decide to build the helix, the roadbed will vary in thickness from a half-inch to an inch, possibly more. If the absolute minimum clearance over the rail you need is three inches and the roadbed is an inch thick, you should figure on four inches of grade change minimum per revolution. Since grades combined with curves produce more drag on the train than either alone, I would try and avoid going steeper than 2% even though the track in the helix is considered concealed track. So when you combine the vertical geometry with the horizontal geometry you get something like this . . .

2.5% grade . . . . . 3.5 inch rise . . . . . 22.5 inch min. radius

2.5% grade . . . . . 4.0 inch rise . . . . . 25.5 inch min. radius

2.0% grade . . . . . 3.5 inch rise . . . . . 28 inch min. radius

2.0% grade . . . . . 4.0 inch rise . . . . . 32 inch min. radius

2.0% grade . . . . . 4.5 inch rise . . . . . 36 inch min. radius

and so on. My point is that you have to remember to consider both the vertical and horizontal geometry in order to establi***he minimum radius for your helix. Good Luck - Ed